Electrochemical fuel cells are known for their ability to produce electricity and a subsequent reaction product through the interaction of a fuel being provided to an anode and an oxidant being provided to a cathode, thereby generating an electrical potential between these electrodes which results in a current flow when an external load is applied. Such fuel cells are very useful and sought after due to their high efficiency, as compared to conventional combustion systems, and the environmentally friendly reaction by-products, such as water, that are produced. Fuel cells, which are classified by electrolyte type, operate under different temperature constraints, as will be discussed shortly. As a result, a problem may arise in lower temperature fuel cells due to the affinity that the catalysts have in adsorbing, among other substances, carbon monoxide (hereinafter, CO) which often is a constituent in the fuel supply.
Electrochemical fuel cells typically employ hydrogen as the fuel and oxygen as an oxidant where, as noted above, the reaction by-product is water. One type of electrochemical fuel cell employs a membrane consisting of a solid polymer electrolyte, or proton exchange membrane (PEM) disposed between the two electrodes formed of porous, electrically conductive sheet material--typically a carbon fiber paper substrate. The PEM has a catalyst layer formed thereon at the membrane-electrode interface so as to promote the desired electrochemical reaction.
In operation, hydrogen fuel permeates the porous electrode material of the anode and reacts with the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the membrane to the cathode and the electrons flow through an external circuit to the cathode. At the cathode, the oxygen-containing gas supply also permeates through the porous electrode material and reacts with the hydrogen ions and the electrons from the anode at the catalyst layer to form the by-product water. Not only does the ion exchange membrane facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen fuel from the oxygen-containing gas oxidant.
Conventional fuels cells have the ion exchange membrane positioned between two gas-permeable, electrically conductive plates, referred to as the anode and cathode plates. The plates are typically formed from graphite, a graphite composite, or the like and may be porous or dense. The plates act as a structural support for the two porous, electrically conductive electrode substrates, as well as serving as current collectors and providing the means for carrying the fuel and oxidant to the anode and cathode, respectively. They are also utilized for carrying away the reactant product water during operation of the fuel cell. When channels are formed within these plates for the carrying of fuel or oxidant, they are referred to as fluid flow field plates. When these plates simply overlay channels formed in the anode and cathode porous material, they are referred to as separator plates. The plates furthermore may have formed therein reactant feed manifolds which are utilized for supplying fuel to the anode or, alternatively, oxidant to the cathode. They also have corresponding exhaust manifolds to direct unreacted components of the fuel and oxidant streams, and any water generated by the reaction, from the fuel cell. Alternatively, the manifolds may be external to fuel cell itself, as shown in commonly owned U.S. Pat. No. 3,994,748 issued to Kunz et al., incorporated herein by reference in its entirety.
Multiple electrically connected fuel cells consisting of two or more anode plate/membrane/cathode plate combinations are referred to as a fuel cell stack assembly (CSA). A cell stack assembly is typically electrically connected in series.
The catalyst is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. The efficiency of these catalyst layers are greatly diminished by contaminants which may adhere to the surface of the catalyst and thereby block the adsorption and reaction of the hydrogen fuel. In particular, CO exhibits a high rate of adsorption onto the platinum catalyst at temperatures below approximately 150.degree. C. and effectively poisons the hydrogen reaction at similarly low temperatures.
The use of fuel cells with proton exchange membranes (PEM), sold by DuPont under the tradename NAFION.TM., is particularly widespread. These fuel cells are especially sensitive to CO poisoning as such cells must contain significant amounts of water and therefore cannot typically operate at temperatures over 100.degree. C., while preferably being operated at approximately 80.degree. C. At these temperatures, CO strongly adsorbs onto the catalyst surface to quickly diminish the performance of the PEM fuel cell. It has been determined that a fuel stream having a concentration of CO being approximately 0.05% (500 ppm) or larger causes an unacceptable performance loss for currently known PEM fuel cells.
It is known to supply a fuel cell with pure hydrogen gas as a fuel, thereby ensuring that no CO performance losses will occur. These systems, however, suffer from the expense and difficulty of producing and storing pure hydrogen gas in quantities sufficient for extended operation. The alternative to a pure hydrogen supply is to produce a hydrogen-rich gas stream where the fuel supply is an easily compressible gas or liquid hydrocarbon such as methane, natural gas or gasoline. This conversion of a hydrocarbon fuel into a hydrogen-rich gas stream is generally accomplished through the use of a steam reformer and a shift converter in combination, effectuating a reduction in the concentration of CO to approximately 0.2% (2000 ppm).
It has been known to further reduce the CO levels in the fuel stream through the injection of (O.sub.2) oxygen or air into the hydrogen-rich fuel stream, an example of which is shown in U.S. Pat. No. 5,432,021, as well as disclosed by the assignee of the present invention within U.S. Pat. No. 5,330,727, incorporated herein by reference in its entirety.
The addition of oxygen oxidizes the CO and works to prevent the poisoning of the fuel cell catalyst. It has been found that at least three chemical reactions take place when O.sub.2 is injected in this way, as shown by the following chemical equations:
1] CO+1/2 O.sub.2 =CO.sub.2, the desired reaction whereby the CO is oxidized into carbon dioxide; PA1 2] H.sub.2 +1/2 O.sub.2 =H.sub.2 O, an undesired reaction of hydrogen fuel into water; and PA1 3] CO.sub.2 +H.sub.2 &lt;=&gt;H.sub.2 O+CO, another undesirable reaction termed a reverse water-shift reaction producing both water and CO.
It should be noted that in this oxidizing process, the prevalent reaction is for the CO to react with the injected O.sub.2, as seen in equation 1. When the amount of CO becomes depleted along the length of the reaction chamber or catalyst bed, the injected O.sub.2 reacts with the existing H.sub.2, thereby consuming the majority of the remaining oxygen, as seen in equation 2, and undesirably depleting the amount of available fuel through the production of water. Additionally, in the now oxygen-poor environment, a reverse water-shift reaction takes place whereby the resultant CO.sub.2 and remaining H.sub.2 react according to equation 3, increasing once again the concentration of CO. For this process to be useful, the temperature of the PEM fuel cell must be carefully regulated. Higher temperatures result in faster reaction rates and more quickly give rise to the undesirable reactions in equations 2 and 3. Lower temperatures may not provide enough of a conversion of CO to CO.sub.2 to enable the fuel cell to operate. A balancing of temperature and fuel cell size must therefore be arranged. It is also necessary to provide some heat exchange system whereby the heat generated during the reaction process is extracted so as not to adversely effect the fuel cell operation.
With the forgoing problems and concerns in mind, it is the general object of the present invention to provide a treatment process for a fuel cell which overcomes the above-described drawbacks, as well as to affirmatively maximize the efficiency of the fuel cell in light of the heat generated during the reaction process.